This application is a continuation-in-part of my copending application Ser. No. 476,024, filed on June 3, 1974, now abandoned, which is in turn a continuation-in-part of my copending application Ser. No. 474,296, filed on May 29, 1974, now abandoned.
This invention relates to the art of electrostatographic copying, an electrostatographic photoreceptor and to a method of treating such a photoreceptor for use in electrostatographic copying machines. This form of copying, originally disclosed by C. F. Carlson in U.S. Pat. No. 2,297,691, involves, as an initial step, the uniform charging of a plate or drum comprised of a conductive substrate normally bearing on its surface a non-conductive barrier layer which is covered by a photoconductive insulating material. This is followed by exposing the plate or drum to activating radiation in imagewise configuration which results in dissipation of the electrostatic charge in the exposed areas while the non-exposed areas retain the charge in a pattern known as the latent image. The latent image is developed by contacting it with an electroscopic marking material commonly referred to as toner. This material is electrostatically attracted to the latent image which is, by definition, in the configuration of the portions of the photoreceptor which were not exposed to the activating radiation. The toner image may be subsequently transferred to paper and fused to it to form a permanent copy. Following this, the latent image is erased by discharging the drum and excess toner cleaned from it to prepare the drum for the next cycle.
The photoconductive insulating material is characterized in that it has a comparatively high electrical resistance in the dark which resistance decreases significantly upon exposure to activating radiation. Both organic materials, such as 2,4,7-trinitro-9-fluorenone in poly(vinylcarbazole), and inorganic materials, such as amorphous selenium, have been successfully used as the photoconductive material in electrostatographic copiers. It has been found to be desirable in some instances to overcoat the photoconductive material with certain substances either to protect it from physical damage or to facilitate the removal of toner during the cleaning step. Typical of overcoating materials are organic resins which form dielectric films and are transparent or translucent to the activating radiation. Overcoatings known to the art have not generally been regarded as contributing directly to improved copy quality. The present invention involves overcoating a photoreceptor with a material which not only protects the photoconductive material from abrasion and chemical contaminants in the ambient, but also enhances the quality of copies prepared from the overcoated photoreceptor. Copy enhancement is related to the formation of increased copy contrast and the reduction or elimination of the undesirable situation in which the copy suffers from background development.
Amorphous selenium has many desirable properties which render its use highly beneficial in electrostatographic copying machines. However, the advent of high speed copiers which make only one or two copies per drum revolution has necessitated the use of photoconductive materials which discharge at a faster rate than pure selenium. This is the case since the increase in copying speed is obtained by increasing the speed of drum revolution. Accordingly, the photoreceptor must go through the charge, expose, develop, transfer, discharge and clean cycle very rapidly. Researchers have discovered that the speed with which selenium can be put through this cycle can be increased by combining the selenium with arsenic to form an alloy. In addition, the use of selenium/arsenic alloys results in a photoreceptor which is sensitive to longer wavelengths of light. This concept is more fully disclosed by Ullrich in U.S. Pat. No. 2,803,542. Thus, selenium alloys containing from about 0.3 percent up to about 48.7 weight percent arsenic are advantageously used as the photoconductive material in high speed electrostatographic copiers. These arsenic containing selenium alloys have been found to suffer from a drawback which has come to be known as print deletion. This phenomena is observed as blank areas in the copy which get progressively larger until the copy quality becomes unacceptable. It is not fully understood what causes print deletion, but it is believed that materials found in the ambient of some locations where copiers are placed contain constituents which react with the arsenic in the photoconductor to form a conductive reaction product which allows the charge to dissipate immediately after the charging step. As a result, no latent image is formed in these areas and print deletion results.
Another drawback associated with the use of selenium/arsenic alloys as the photoconductive material in electrostatographic copying is long recovery time from light fatigue. Great care must be exercised to prevent the alloy from being exposed to light because such exposure will render the material non-photoresponsive for relatively long periods of time.
It would be desirable and it is an object of the present invention to provide a novel overcoating for electrostatographic photoreceptors.
An additional object is to provide such an overcoating which enhances the quality of copies produced by the overcoated photoreceptor.
Another object is to provide such an overcoating which reduces or eliminates the problems of copy deletion and light fatigue recovery associated with the use of selenium/arsenic alloys as photoconductors.